What is the most efficient way to model this kind of wrought-iron shape?
This is no shortcut. But you will have detailed control over your model.

I would start with a highres UVsphere and then add geometry with snap and shrinkwrap to follow the spheres surface.



Then add a solidify modifier followed by a subsurface modifier:



You can apply the solidify modifier and fiddle with the geometry if you want to make it beveled or such.